An electric vehicle (EV) is a type of vehicle that is driven by a battery powered motor. As a result, the vehicle has fewer pollution sources, such as exhaust gas and noise, than a conventional gasoline engine vehicle, fewer failures, a longer lifespan, and simple driving operation.
EVs are typically classified into a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and an electric vehicle (EV) according to the driving source. The HEV has an engine as a main power source and a motor as an auxiliary power source. The PHEV has a motor as a main power source and an engine mainly used when a battery is being discharged. The EV, meanwhile, does not have an engine, and has a motor as its sole driving source.
In order to charge a battery equipped in the EV using a wireless charging method, it may be necessary to couple a primary coil of a charging station with a secondary coil of the EV using magnetic resonance. In order to improve efficiency of wireless charging, it may be necessary to align the primary coil with the secondary coil. In a magnetic resonant wireless power transfer system, if the primary coil and the secondary coil are not aligned, the efficiency of wireless power transfer may degrade drastically.
For example, as shown in FIG. 1, when the EV has a secondary coil (referred to as “Rx coil”) magnetically coupled to a primary coil (referred to as “Tx coil”) installed on the ground, the secondary coil should be aligned with the primary coil. Otherwise, for example, as the distance (a) increases between a first extension line, extended along the central axis of the Tx coil in the form of a circular ring, and the second extension line, extended along the central axis of the Rx coil in the form of a circular ring, the efficiency of power transfer may decrease drastically. Here, as an example, the diameter of the primary and secondary coil may be assumed to be 300 mm, and the distance (d) between them may be assumed to be 105 mm. As shown in FIG. 2, the efficiency of power transfer (represented with S-parameter S21) in the above-described wireless power transfer system having the primary coil and the secondary coil is drastically reduced from −2.5 dB to −22.5 dB, as the distance (a) between the center axes of the Tx coil and the Rx coil is increased from 0 to 450 mm by 50 mm.
As described above, in an EV wireless power transfer (WPT) system, the alignment between a transmission coil and a reception coil greatly affects the efficiency of the wireless power transfer. Coil alignment may thus become an important requirement for high efficiency wireless power transfer. Accordingly, various research and development related to coil alignment are underway.
Meanwhile, in the WPT system of the EV, the wireless power transfer is performed between the transmission pad installed in the parking area and the reception pad mounted on the EV. Therefore, the coil alignment of the EV WPT system is difficult as compared to cases of mobile devices and charging pads for them. For this reason, a method of performing coil alignment on the basis of a position showing a maximum magnetic field coupling coefficient above a reference value by using an auxiliary coil or the like has been suggested. However, there is still a shortage of effective coil alignment methods applicable to the EV WPT systems. Thus, there is a need for a new coil alignment scheme for the EV WPT systems.